It is known in the art that many plastics materials, such as polyethylene film, can be exposed to a high voltage gaseous discharge having corona characteristics for the purpose of improving the capacity of the surface of such materials for the adherent reception thereupon of printing inks, or to improve the adhesion characteristics of such materials for various substrates. In general, a corona discharge is a high voltage, low current phenomenon with voltages typically being measured in kilovolts and currents in milliamperes. Broadly stated, a corona-type discharge is produced by capacitively exciting a gaseous starting material lying between two spaced electrodes, at least one of which is insulated from the gaseous medium by a dielectric barrier. There are many different types of generators on the market for producing a corona-type discharge. Most of the commercial generators employ alternating current supplied at frequencies up to 500 kHz or higher. Gap voltages up to 15 kv or more are employed to effectively treat a polymer film which can be continuously passed through the gap at speeds up to 500 feet per minute or more. In practice, an energy density-to-film surface of the order of about 1-4 watt-minute per square foot of film surface has been used to improve the affinity of the surface of a polymer body for adhesives, inks and other polar adherends.
U.S. Pat. No. 3,736,493 to Louis A. Rosenthal et al. discloses that an alternating-directional, sonic frequency electrical voltage employed for film treating may be of a broad range of sonic frequency which can be varied to effect surface treatment under maximum loading conditions. Thus a treating system capable of broad frequency variation of treating voltage over a range of 20 to 20,000 Hz can be provided.
U.S. Pat. Nos. 3,736,492 and 3,736,494 to Louis A. Rosenthal et al. disclose methods and apparatuses for treating plastic film which entails exposing the plastic film to a high intensity voltage accompanied by corona discharge. The high intensity voltage disclosed in these patents comprises a series of alternating-directional sonic frequency pulses of electrical voltage which are developed in a pulse forming circuit.
Other United States patents disclosing corona generators and/or film treating methods using a corona discharge are U.S. Pat. No. 3,496,092 to James C. Fraser, U.S. Pat. No. 3,294,971 to Jack C. von der Heide and U.S. Pat. No. 3,729,672 to Louis A. Rosenthal. Thus it is known in the art that subjecting a polymer film to a corona discharge using an energy of up to 1-4 watt-minute per square foot can increase the surface adhesion characteristics for such film to a satisfactory degree.
It is also known that a coating of low-density polyethylene film can be fused or bonded onto a steel or aluminum substrate by heat-treatment techniques. For example, polyethylene film can be deposited onto a metal substrate heated above the melting point of polyethylene and then subjecting the laminate to a further heat treatment of above 140.degree. C. to bond or fuse said polyethylene film to the substrate. Although this method can be employed to bond or fuse a film of polymer onto a metal, it does require a dual heating procedure whereby the second heating step is conducted while the film is in contact with the metal. In applications where the metal substrate cannot be exposed to a heat treatment, this method could not be employed.
U.S. Pat. No. 3,754,117 to Jacques Walter discloses an apparatus for the corona treatment of a layer of plastic material whereby the plastic material, when it is being extruded from a conventional extruder, is heated to the highest temperature which it is capable of withstanding, and while heated to such a temperature the plastic material is subjected to a corona discharge. This method of treating plastic materials requires an apparatus having means that will maintain the plastic materials at the highest temperature they can withstand while simultaneously providing means for producing a corona discharge for such heated plastic materials.
It is a primary object of this invention to provide a heat-treated, corona treated polymer body having a surface with improved adhesion characteristics for adhesives and the like.
Another object of this invention is to provide a heat-treated, corona treated polymer bonded to a metal substrate by a conventional adhesive.
Another object of this invention is to provide a method for improving the surface adhesion characteristics of polymer bodies.
Another object of this invention is to provide a heat-treated, corona-treated polypropylene film having excellent surface adhesion characteristics for adhesives and, therefore, admirably suited for bonding to metals or other type substrates via the adhesives.